This divergent cone which begins at the top of the tuyere zone has received attention at the hand of a vast army of engineers and furnacemen, and many solutions of the problem of a suitable construction have been attempted. The divergent shape of this portion of the structure and its relative lack of solid foundation upon which to build a patch if it fails, combined with the scouring action of the blast traveling up it, and of the semi-reduced iron and scouring slag traveling down it, have made the problem of maintenance of this part of the furnace a difficult one.
One of the most commonly used constructions is that shown by the upper portion of Fig. 164. It consists of several rows, in this case eight, of cooling plates, those in one horizontal row alternating in vertical location with those in the rows above and below it. In other words, the plates in each of the eight horizontal rows are not vertically above one another, but are staggered for reasons of mechanical strength, because the plates are very much weaker against compression stresses than the brickwork, and if they were all in one row might be crushed by the expansion of the bosh walls due to the heat. By having them in alternating rows larger masses of brickwork intervene between them, making a sort of series of arches which do not readily crush. Moreover, if all these cooling plates were arranged in vertical rows, the brickwork between these being relatively without protection would be scoured out by the erosive actions I have mentioned.
By staggering the plates vertically the space between a pair in one row is protected to a great extent by the plates in the other rows above and below that space. This construction is shown in Fig. 165 (page 248). Just above the bustle pipe portions of two rows of cooling plates and their vertical staggering may be quite clearly seen.
The cooling plates must be protected from the weight of the brickwork above them and this is done by one of two methods. Either a flat arch is turned in the brickwork above them or a cast-iron box is built into the brickwork which projects in from the outside of the wall about one-half or two-thirds the length of the cooling plate, which is inserted through and protected by this box. Both methods are good but the brick arch is much cheaper and just as good for all practical purposes, so is to be preferred. Circumferential strength is supplied by very heavy bands of steel running around the bosh between the rows of plates.
This makes a good, strong, substantial construction, but in addition to being very expensive on account of the great quantity of copper castings required, it is liable to one serious fault. This is that the erosional influences I have mentioned cut out the brickwork between the plates and leave the noses of the latter sticking out into the furnace almost like a flight of steps. I have tried to illustrate this action by the dotted lines in Fig. 164 and can state most emphatically that the conditions shown by this sketch represent only a fraction of the erosion that I have seen in actual practice after a campaign of only a year or two.
When the action has proceeded to the extent indicated or further, the shelves formed by the projecting noses of the cooling plates make ideal lodging places for half-molten slag and such materials as it cements together, and such a mass builds up on the noses of the plates at times with considerable rapidity. Then a change in the condition of the furnace comes about or the accumulations are no longer able to maintain themselves on the narrow footing offered by the cooling plates, but in either event the whole mass sloughs off into the hearth and having no time to be treated by the heating and reducing action of the ascending gas is precipitated into the hearth in a half-raw condition with resulting ruin to the iron and slag already formed therein, and the partial chilling of the furnace, perhaps to a serious extent.
Fig. 16G. Segmental bosh jacket built by Birdsboro Foundry & Machine Company.
It is these steps and the consequent irregularity of the work of the furnace at times which has led some furnacemen to abandon this type of construction for one whose wearing surface should be smoother and more regular. This result has been sought in some cases by building up a water-cooled bosh jacket of cast-iron segments as shown in Fig. 166. These segments as shown are strongly bolted together to supply the necessary strength to resist the pressure, and then receive a thin lining of brick, its thickness depending upon the opinion of the furnacemen as to what is necessary, but commonly from nine to eighteen inches. This is an expensive construction and requires careful machining throughout, because if the least crack exists the gas will blow out through it with destructive effect and on account of the crack being small it is almost impossible to get anything in the nature of a clay stop to stay in it.
A third type of construction consists of a bosh jacket made entirely of steel plates and cooled solely by external sprays. This can be made an absolutely air-tight job of boiler work, thus eliminating any possibility of gas leakage such as I have described, while the cooling by means of a continual film of water on its exterior surface produced by sprays is one of the most efficient forms which can be obtained in any way whatever, a much greater surface being exposed to the cooling action of the water than in the case of the cast-iron built-up jacket.